The electrowinning of metals from solutions thereof, particularly acidic solutions, is a well-known commercial process. In general, the acidic solutions employed in such electrowinning processes are obtained by treating ores or ore concentrates with acidic leaching solutions, usually sulfuric acid solutions, which sometimes are concentrated by a solvent extraction process. The leach liquor is then electrolyzed within an appropriate electrochemical cell. During the electrolysis of the leach liquor, large amounts of oxygen are evolved at the anode necessitating the employment of high input voltages to overcome the oxygen overvoltage, thereby detrimentally affecting the economics of such electrolytic processes.
In order to reduce the energy consumption required in electrowinning processes, it has been proposed to equip the electrolytic cell with a fuel fed porous catalytic electrode. There are problems with such a process, however. For example, the metals contained in the solution having oxidation potentials below that of hydrogen are deposited on the porous anode, thereby deactivating the anode catalyst. Moreover, the deposition of a coherent film of the metal being electrowon from the solution effectively prevents the flow of electrolyte through the pores of the anode, thereby terminating the electrochemical process. Therefore numerous techniques have been proposed for preventing metal depositions, e.g. copper deposition; on such electrodes. Illustrative of such techniques are those disclosed in U.S. Pat. No. 3,103,473, U.S. Pat. No. 3,103,474, and U.S. Pat. No. 3,793,165.
In contrast to electrowinning, electrorefining processes typically employ a soluble anode which is composed principally of the metal which is to be deposited on the cathode. Thus, for example, in the electrorefining of copper, an anode which is composed largely of copper, but may contain other metals as contaminants, is employed. The presence of other metal contaminants can be tolerated provided they are not electrodeposited with the copper during the plating operation. Examples of electrorefining processes are disclosed in the following: U.S. Pat. No. 1,449,462, U.S. Pat. No. 3,994,789, and U.S. Pat. No. 4,207,153.